Bass creep: Could smallmouth threaten the Yellowstone River’s iconic trout?

LIVINGSTON, Montana – They launched the raft close to dawn and rowed down the Yellowstone River, trying to move quickly.

The three U.S. Geological Survey researchers on board had a few stops to make on this river east of town. They wanted to be done by early afternoon, before the August sun got high and hot.

They reached the first of three stops by about 7:40 a.m. Maddie McKeefry, a fisheries technician, and Robert Al-Chokhachy, one of the scientists overseeing the whole project, headed for three nets in a side channel on the south side of the river.

Hundreds of miniature fish had schooled up inside the nets overnight. Now it was time to see what they were. The scientists dumped the fish into a live well – essentially a mesh clothes hamper sitting in the water. McKeefry sat on a bucket and sorted small groups of the fish on a white PVC pipe cut in half with a ruler in the center. Al-Chokhachy stood nearby with a clipboard and wrote down what McKeefry found.

Most of the fish were suckers and dace, so small the only way to measure them was with millimeters. But McKeefry kept an eye out for the interloper that inspired this whole effort to begin with: smallmouth bass.

Smallmouth bass are known to live in the lower reaches of this river, but there has been talk of the warm-water species inching upstream. The fish are caught somewhat regularly on the Yellowstone downstream of Big Timber. There have been one-offs caught as far upstream as Emigrant – dead center in the trout nirvana known as Paradise Valley.

Some fear climate change is only going to make the famous trout stream more habitable for the non-native predator, which is known to feed on juvenile fish. If bass were to establish upstream, the fish would become another competitor for food and a potential consumer of young trout, adding stress to a cold-water species already dealing with making a living in a warmer world.

For this project, a collaboration between the USGS, Montana State University and Montana Fish, Wildlife and Parks, researchers have spent four years looking at hundreds of miles of river from the Paradise Valley to eastern Montana. They’re studying where smallmouth are and where they could go based on what food and habitat is available.

Al-Chokhachy said the river already has favorable temperature conditions for bass to move upstream, and that it’s only a matter of time.

“This is the longest undammed river in the Lower 48,” he said. “It’s great, but it also means there’s nothing stopping these fish from moving up.”

‘You’re going to see bass following’

Water warms slower than air, but it still warms. The 2017 Montana Climate Assessment said the state’s rivers and streams have seen a temperature increase over the decades alongside the increase in air temperatures.

Temperatures are highest in late summer, when streamflows are lowest. Low flows make the water bodies more sensitive to air temperatures, linking warming rivers to long-term trends in snowpack.

That combination regularly prompts time-of-day fishing closures in the late summer for many of the state’s famous streams in an attempt to protect trout. The closures are triggered after three consecutive days that the water temperature peaks at 73 degrees. Wildlife officials approved a permanent evening closure from July 15 to Aug. 15 for the lower Madison , a first for the state.

High temperatures have also helped cause fish kills in the past. The biggest in recent memory came in 2016, when thousands of whitefish were found dead in the Yellowstone as the result of low flows, high temperatures and a parasite.

The rise has been happening for a long time, said Daniel Isaak, a U.S. Forest Service scientist. He estimated that mean stream temperatures in the late summer are about 1.8 degrees higher than they were historically. He said a similar rise is inevitable by the middle of this century, and that temperatures could go higher after that.

“We’re interpreting the long-term river and stream increases as being a symptom of anthropogenic climate change,” Isaak said. “If that’s the case, the models are suggesting we’re on a path to continue warming through at least mid-century.”

What that means is vast and far-reaching, Isaak said. It could make life tougher for cold water salmonids like native trout and whitefish. It could make small streams better for fish like brown trout, which can survive warmer water.

And it could make rivers like the Yellowstone better for bass.

“The thinking is that as things warm up and you see that critical temperature inching upstream along rivers, you’re going to see bass following,” Isaak said.

Al-Chokhachy is part of a team of researchers that is looking at the impacts on the region’s rivers and its fish populations. He and Adam Sepulveda, another USGS scientist, did the baseline work to get the bass study going. Now they have a graduate student working on it, and they expect the student will have something ready to publish in a journal in January.

It’s one of several climate-related projects they’ve done over the last several years. They published a study last year on how brown trout moving into a small stream in the Crazy Mountains affect the Yellowstone cutthroat trout that rely on the stream to spawn. They’ve also looked at changing runoff patterns across the ecosystem, finding that peak runoff is arriving earlier and that late summer flows are getting lower.

“We’re trying to characterize the sensitivity of these ecosystems and these species to a changing climate,” Al-Chokhachy said. “And the second part of that is, ‘What can you possibly do?’ ”

A surprise

Dan Vermillion, owner of Sweetwater Travel in Livingston, grew up in Billings and has spent a lot of time fishing the Yellowstone. His family has a ranch on the river near Big Timber where he runs a guide school. He doesn’t recall seeing bass there as a kid, but now his guide school averages one a week.

“It’s just a surprise,” he said. “You expect Montana and you expect the Yellowstone, at least in the upper reaches, to be a trout fishery.”

The bass his guides have caught weren’t stocked in the Yellowstone. The species, first stocked in Montana in 1914, was never put directly into the river. Instead, bass moved in after being introduced in the Tongue and Bighorn rivers in southeastern Montana. For the most part, the species has stayed in the Yellowstone’s eastern reaches.

Smallmouth are considered a warm-water fish, but biologists think of them more as a “cool-water” fish. Their temperature comfort zone overlaps with that of trout. The lower end of their comfort zone is about 59 degrees, Al-Chokhachy and Sepulveda said, which is toward the warm end for some trout species.

That combined with research elsewhere suggests conditions are ripe for bass in much of the Yellowstone already, Al-Chokhachy said. By weighing the suckers and dace they find in the nets, they’ve found there’s plenty of bass food throughout the river, even without including young trout. Sepulveda saw crayfish close to Livingston on one float this year, much farther upstream than the organisms are normally seen.

It raises the question of why smallmouth aren’t already more common in the upper part of the Yellowstone. A few individual explorers have been caught over the years, but researchers have only found juvenile bass east of Big Timber. They’ve only seen juvenile bass near Big Timber once – in 2016.

That indicates to them that spawning, which is key for the species to establish somewhere, is only happening farther downstream. Part of their research looks at why.

There are a few theories. Upstream, there aren’t as many side channels, which provide better habitat because the water is often warmer. Density may be a factor, meaning the fish might not move up without being crowded out of a place first.

Or it could all be temperature driven. It could be that winter water temperatures dip below what’s comfortable for bass, making it tough for them to survive.

That’s what they’re looking at closely in eastern Montana. They’re doing more intensive sampling to gauge the size of the bass and whether the fish can put on enough fat during summer to survive winter. Long winters are hard on them.

But if that’s what’s limiting their spread, Al-Chokhachy wonders how long it will hold.

“The thing is, if it’s the duration of winter, that’s the thing we know is changing the fastest with climate,” he said. “Summer is getting warmer, yeah, but not really as fast as spring and fall, and the cold colds are also what’s going away.”

The upper Yellowstone is still pretty cold compared to some other streams in the region. That’s one reason Travis Horton, the regional fisheries manager for Montana Fish, Wildlife and Parks, isn’t as concerned about smallmouth taking over the Yellowstone in the short term as he would be about an illegal introduction of the fish into the lower Madison or Jefferson rivers.

Temperature data shows those two rivers regularly reach much higher temperatures than the Yellowstone. Horton expects smallmouth could thrive there immediately after an introduction, which could allow them to run farther up the tributaries that make up the headwaters of the Missouri.

It’s not that he doesn’t see bass as a threat in the Yellowstone – their existence certainly isn’t good for trout, he said. It’s that he thinks the upstream crawl will take time.

“I expect we’ll see a creep over years,” Horton said. “We won’t see an establishment of large numbers probably for the rest of my career.”

Sepulveda said they hope that’s right.

“Hopefully, they won’t be a problem for 10, 20, 30 years,” Sepulveda said. “That gives us 10, 20, 30 years to figure out the potential best practices to make sure it never gets to that point.”

How long can a trout fishery continue to exist?

Sometime just after noon, the three reached their final net site for the day, not far from the takeout west of Springdale. The raft was loaded with the other nets. They planned to set them in a new spot that night.

The last stop was a wide algae-strewn side channel. While McKeefry and Al-Chokhachy headed for the nets, the third member of their group, Patrick Hutchins, took a small machine to the main channel to grab water samples.

He’d done this at the other stops, too. The samples are used for Environmental DNA testing, or eDNA for short. The method scans the sample for DNA particles. It can detect evidence of a fish researchers wouldn’t see otherwise. In this case, they’re using it as another line of evidence to make sure they’re not missing bass.

A few drift boats with anglers floated by as Hutchins grabbed the samples. One asked what he was doing, and he explained he was checking for any sign of smallmouth bass.

“Their last question to me as they were floating by was, ‘Is it a good thing or a bad thing that they’re here?’ ” he said. “And I just told them it depends on who you ask.”

There are varying responses to the idea of bass moving up the river. Some anglers would be apoplectic if smallmouth started displacing trout, or had any serious impact on them. Others wouldn’t mind some variety. The researchers have heard from all of them.

Vermillion, for his part, said fishing the river now offers something unique, and that bass aren’t it.

“There’s only one place in the world you can catch Yellowstone cutthroat, and that’s in the Yellowstone drainage,” Vermillion said. “You can catch bass in most parts of the world.”

He said bass aren’t inherently bad. Some people do love catching them. But they are another mouth competing with trout for food, a potential predator and a sign of climate change.

“It’s more a function of being a sign of changing water temperatures,” Vermillion said. “All of us who care about a trout fishery have to ask the question of how long a trout fishery can continue to exist and survive in the face of warming temperatures.”

The last net of the day was in deep mud. They shook out the fish and sorted them, again keeping an eye out for the interloper. They were still well upstream of where they’ve seen young fish before. A juvenile bass here would be news.

But there wasn’t one. Sculpin, suckers and dace, but no bass. They’re still waiting on the eDNA results from the trip.

Al-Chokhachy said not finding bass is just as important as finding them. It confirms their theory that the spawning is still to the east, that the upstream creep isn’t here yet.

“For the trout, it’s good news,” he said. “It really is.”